The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
What Happens on Mounjaro Day 1: Your First Injection Guide
Starting Mounjaro (tirzepatide) is a significant step in your weight management journey. On day 1, you'll receive your first 2.5 mg dose — the standard starting point for all UK patients. This initial injection is smaller than the maintenance doses you'll eventually progress to, allowing your body to adjust to this dual GIP and GLP-1 receptor agonist medication. Understanding what to expect, how the injection works, and how Mounjaro differs from alternatives like Wegovy will help you feel confident and prepared. Use our free GLP-1 weight loss calculator to explore potential outcomes as you begin treatment.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Your First Mounjaro Injection Works: Mechanism vs Wegovy
Mounjaro's dual mechanism is what sets it apart on day 1 and beyond. Unlike Wegovy (semaglutide), which activates only the GLP-1 receptor pathway, Mounjaro activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 receptors simultaneously. This dual action means your appetite signals are influenced through two separate pathways from your first injection onwards.
- GLP-1 activation: reduces appetite, increases fullness, slows gastric emptying
- GIP activation: helps regulate fat utilisation and glucose metabolism
In the SURMOUNT-1 clinical trial, participants on Mounjaro lost an average of approximately 15% body weight at the 5 mg maintenance dose, with further weight loss at higher doses (10 mg and 15 mg). Wegovy, by contrast, showed average weight loss of around 15% in STEP-1 trials. Both require a reduced-calorie diet and increased physical activity — the injection alone is not sufficient for results.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Day 1 Side Effects and Suitability: Which Medication Is Right for You?
On your first day, you may experience some side effects, though many people tolerate the 2.5 mg starting dose well. Most common early side effects include mild nausea, constipation, and reduced appetite. These typically decrease over the first 2–4 weeks as your body adjusts. More serious side effects (severe stomach pain, gallbladder problems, allergic reactions) are uncommon but require immediate medical attention.
Whether Mounjaro or Wegovy is suitable for you depends on your individual health profile, which is why a UK-licensed prescriber assesses you before day 1. Both require a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Your prescriber may recommend one over the other based on your medical history, concurrent medications (especially oral contraceptives), and treatment goals. Try our BMI calculator above to confirm your starting eligibility context.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Will I feel anything on Mounjaro day 1?
Most people report minimal symptoms on the first 2.5 mg dose. Some experience mild nausea or appetite suppression within hours, others feel no immediate change. Effects accumulate over days and weeks as the medication builds in your system. Side effects typically peak around days 2–3 and improve thereafter.
How is Mounjaro day 1 different from starting Wegovy?
Both start at a low dose to allow adjustment. Mounjaro starts at 2.5 mg (dual GIP+GLP-1), while Wegovy starts at 0.25 mg (GLP-1 only). Mounjaro's dual mechanism may produce slightly different side effect patterns. Clinical trial data shows both support significant weight loss when used alongside diet and exercise — results vary by individual.
Can I eat normally on my first day of Mounjaro?
Yes, you can eat normally immediately after your injection. However, many people report reduced appetite within hours, leading to naturally smaller portions. This appetite suppression is the intended effect. You should still aim to eat nutritious, balanced meals — the medication is designed to support, not replace, healthy eating habits.
What should I do if I have severe side effects on day 1?
Severe side effects (severe stomach pain, signs of allergic reaction, chest pain) require immediate medical attention — contact NHS 111 or go to A&E. Mild nausea and constipation are common and usually improve within days. Your prescriber can advise on managing these. Never stop the medication without medical guidance.
How soon will I see weight loss results after day 1?
Weight loss typically becomes noticeable after 2–4 weeks as you progress through dose titration and your body adjusts to the medication. The 2.5 mg starting dose is deliberately low — meaningful weight loss usually occurs once you reach maintenance doses (5 mg or higher). Individual results vary significantly. Track your progress with our free progress tracker.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.